Sometimes it's also 2 clients that are in 1 location.
It's no longer available but I used to regularly visit a coffee shop in a major book store and shop each of them for different MSCs.
Here in CA, parking shops are popular, so when I have a bar or dinner shop, I can often find a parking shop for the valet at the hotel/restaurant or nearby. The valet staff are sub-contractors who are shopped by a different MSC.
The grocery store that makes you an employee also has fast food & smoothie places in some locations. I once managed to work it out that I did all 3 at once.
Each of those create very natural interaction when I'm on the shop, make me look like a regular customer and have no conflict between the clients I'm evaluating.
I took a hotel shop recently that included shopping the valet. I saw a valet-only shop for a different MSC at the same location and asked the hotel MSC about that. They told me that the hotel turned over the valet portion of the report to the valet company, so it would have been a conflict and major faux pas to take both of those, since they valet company would have paid twice for the same shop.
It's best to check with the MSC if you are in doubt.